Hi, RFC 2822 section 3.2.3 states:
There are several places in this standard where comments and FWS may
be freely inserted. To accommodate that syntax, an additional token
for "CFWS" is defined for places where comments and/or FWS can occur.
However, where CFWS occurs in this standard, it MUST NOT be inserted
in such a way that any line of a folded header field is made up
entirely of WSP characters and nothing else.
In section 3.2.6 it defines <unstructured> header field bodies:
unstructured = *([FWS] utext) [FWS]
A prominent example is the <subject> defined in section 3.6.5:
subject = "Subject:" unstructured CRLF
Expanding the [FWS] at the end (ignoring <obs-FWS>) results in:
subject = "Subject:" *([FWS] utext) [[*WSP CRLF] 1*WSP] CRLF
This is not intended and forbidden by a MUST NOT in 3.2.3, the
ABNF for <unstructured> in section 3.2.6 shoukd have this form:
unstructured = *([FWS] utext) (*WSP / obs-FWS)
Kind regards, F.Ellermann